8. Extent to which colonic fermentation of carbohydrates contributes to energy requirements in childhood

8.1. Colonic fermentation

8.2. Energy from SCFA

8.3. Factors influencing SCFA production

8.4. Gross versus metabolizable energy

8.5. Faecal energy and non-starch polysaccharide

8.6. Faecal energy in cystic fibrosis

8.3. Factors influencing SCFA production

The extent of the production of SCFA and the contribution to
metabolizable energy intake appears to depend on several factors.

Firstly, the amount of dietary fibre and the degree to which it is
digested. There has been considerable interest in the digestibility of different
forms of dietary fibre within the human large intestine (CUMMINGS, 1984).
However, the extent to which microbial fermentation takes place and the extent
to which the end-products of fibre fermentation are absorbed and contribute to
metabolizable energy intake remain an area of controversy. Increases in
microbial cell excretion were observed in association with an increased intake
of vegetable fibre from cabbage whereas the increased consumption of the much
less digestible wheat fibre resulted in only a small change in faecal microbial
excretion (STEPHEN and CUMMINGS, 1980). Thus, the higher the fibre intake,
particularly that derived from beans, vegetables and fruits, the greater the
potential contribution from colonic fermentation.

Secondly, the amount of unabsorbed carbohydrate delivered to the
large intestine. Clearly, this may be increased as a result of maldigestion and
malabsorption associated with disease (e.g., cystic fibrosis or lactose
intolerance). However, it is now clear that some foodstuffs contain relatively
large amounts of starch in a form that is resistant to digestion - both in
vitro and by human digestive enzymes. A number of studies have demonstrated
that not only does the resistant starch largely escape digestion in the human
small intestine, but that other types of starch may also pass into the large
intestine to undergo fermentation (CUMMINGS and ENGLYST, 1987).

Thirdly, the quantity and nature of endogenous material delivered
to the large intestine is not clear. Mucus degradation by colonic microflora has
been well documented, and the presence of bacterial sub-populations that produce
extracellular glycosidases with the specific role of degrading complex
oligosaccharides of mucin in the gut lumen have been identified (HOSKINS and
BOULDING, 1981). It has not been possible to directly quantify mucus production
and epithelial cell losses, hence the extent of the contribution made by
fermentation of mucopolysaccharides and glycoproteins to metabolizable energy
remains unknown. However, in circumstances where mucus production is
substantially elevated (e.g., in cystic fibrosis), the potential capacity for
energy to be salvaged might be substantial (see later).

Finally, what is the overall magnitude of the gross energy intake
and to what extent can the intake satisfy the energy needs of the individual?
The relative contribution made by colonic fermentation may be relatively small
when the gross energy intake is high. In contrast, for individuals on a marginal
intake of energy, or in whom requirements are elevated, the relative
contribution will be greater and may be
critical.